OpenGL. The graphics standard known as “OpenGL” defines a software interface for controlling graphics display systems. More specifically, it defines a number of commands that may be issued by client software to cause an OpenGL-compliant graphics system to draw. Some OpenGL commands are designed to place the graphics system into a particular state (e.g., set the current color to a particular color, or set the current model view matrix to a certain value). Other commands are designed to specify primitives for rendering (e.g., define the vertices of a triangle or other polygon).
Display Lists. It is common for client software to issue OpenGL commands in what is called immediate mode so that the commands will be executed upon receipt by the graphics system. But OpenGL also includes a notion of display lists. A display list represents a set of one or more commands. Once created, a display list is stored for possible later or repeated invocation by client software. For example, a display list might be created for rendering a wheel and then invoked four times to draw the four wheels of a car. A display list may contain both rendering commands and state commands. For lists containing state commands, the effect of the state commands persists in an OpenGL-compliant display system after the list has been invoked. It is possible to call a second display list from within a first display list. For further background information on OpenGL and OpenGL display lists, see for example OpenGL Programming Guide (Addison-Wesley, 1993).
Bounding Volumes. One known performance enhancement technique in computer graphics is the concept of a bounding volume. For example, a simple shape such as a cube may be defined to enclose a more complex shape such as a star. In such an example the cube would be referred to as the bounding volume for the star. During rendering the coordinates defining the cube are compared with the coordinates defining a viewing volume. If the cube is outside the viewing volume, then the star is not rendered. It is computationally simpler to compare the viewing volume with a simple shape such as the cube than it is to compare it with a complex shape such as the star. Consequently, the use of bounding volumes helps to conserve resources and improve performance.
Bounding volumes may also be nested. For example, two objects in a scene may each have their own individual bounding volumes defined, and a third bounding volume may be defined to enclose the first two bounding volumes. If the third bounding volume is not in the viewing volume, then both of the objects can safely not be rendered. In such as case, testing the viewing volume against the third bounding volume saves computing resources that would have been required to test the first and second bounding volumes individually. Nested bounding volumes are often referred to as hierarchical bounding volumes.
The Hewlett-Packard Visualize Center SV6. In one OpenGL-compliant graphics system called the Hewlett-Packard Visualize Center SV6, display list logic utilized bounding volumes at two levels: the leaf level and the root level. First, the SV6 system created bounding volumes for each of the primitives defined in a display list. It did so on a primitive-by-primitive basis and stored the bounding volumes in the display list along with the corresponding primitives. This feature enabled culling to be performed on a primitive-by-primitive basis (the leaf level) whenever the display list was invoked. Second, the human user was able to set an environment variable if he wished to operate in a “stateless” display list mode. The implicit promise made by the user when operating in stateless display list mode was that no display lists created by his application would contain any state commands. When in stateless display list mode, for each list, the SV6 system created a single global bounding volume that enclosed all of the primitives in the list (the root level). It did this in addition to creating separate bounding volumes for each individual primitive at the leaf level. When operating in stateless display list mode, an entire list could possibly be culled when the list was invoked, depending on the outcome of testing the global bounding volume against the viewing volume. But stateless display list mode could not be used if even a single one of possibly numerous created display lists would contain a state command.